There used to be one extension, but the way the author achieved the feat was by replacing Flarum's TextFormatter completely, breaking compatibility with most extensions.
I don't know of any library that would provide WYSIWYG functionality with markdown/bbcode source, and that's without counting custom formatting added by extensions. Usually WYSIWYG is HTML-based, which isn't compatible with Flarum TextFormatter approach.
The best strategy I think would be to have one of those hybrid-WYSIWYG editors where you see both the source of the markdown but also contextual formatting. It's not really WYSIWYG anymore but might be more user-friendly for users not familiar with markdown.